1. Field of the Invention
The invention relates to a process for producing pipes, especially large pipes, according to the UOE process.
2. Description of the Prior Art
A process known as the UOE process is the most frequently used method of producing large longitudinally welded pipes (Stradtmann, Stahlrohr-Handbuch [Steel Pipe Handbook] 10th Edition, Vulkan Verlag, Essen 1986, pp. 164 to 167). In this process, after preparation of a longitudinal edge (weld seam bevelling, bending), a U-shaped split pipe is formed from a flat sheet on a bending press (U press). Rounding into a pipe is then carried out on a different press with self-closing dies (O press). Because the pipes, after inner and outer welding, often do not meet requirements for diameter and roundness, they are then calibrated by cold expansion (Expansion).
In expanding pipes with small diameters and thick walls, a problem arises in that the curvature after longitudinal seam welding is insufficiently compensated for by mechanical expansion so that the non-straightness fails to fall within customer tolerances. To solve this problem, it has already been proposed to combine the expander with a straightening press (Iron and Steel Research Lab, Sumitomo Metal Industry Ltd. Vol. 1 (1988) p. 1569). Arranged at some distance (several pipe diameters) in front of and behind the expander head are two additional frames, in which are installed a clamping device, on the one hand, and a path-controlled bending insert, on the other. If the weld seam lies on the top (as is the case in known expanders, in which the uppermost segment of the expander head has a slot to accommodate the weld seam), the welded pipe is always curved before expansion in such a way that the midpoint of the curvature lies above the pipe or expander axis. With the help of the omnipresent support roll for the expander head, a three-point bend is realized. It is disadvantageous in this method that the pipe must execute several expansion steps before it can be grasped by the bending device. The straightening process therefore never encompasses the end areas of the pipes.
The object of the invention is to provide a method of producing pipes according to the UOE process, especially large pipes, that allows the curvature after longitudinal seam welding to be compensated for over the entire pipe length to such an extent that the non-straightness lies within customer tolerances, even in the case of pipes with small diameters and thick walls.
In contrast to the known prior art, the present invention uses cold expansion for straightening as well. In this present inventive method, the pipe to be calibrated is clamped as near as possible to an expander head, and the free pipe end is flexibly deformed. After this, the pipe is cold expanded in the known manner. This procedure is carried out incrementally in speed with the respective expander steps, up to half the pipe length. The pipe is then rotated, and the second pipe length is similarly processed. Instead of rotating the pipe, the second pipe half is similarly processed on a second expander unit in mirrored fashion. The flexible deformation, which takes the form of depression counter to the curvature of the pipe and rotation around the cross axis, produces additional force and additional moment in the clamping area. The superimposition of force, in interaction with the cold expansion by means of the mechanical expander, leads to the desired straightening effect in the area between the cold expansion and the clamping device.
The flexible deformation is established as a presetting, and the amounts thereof decrease incrementally. The degree of pipe curvature found after longitudinal seam welding is used to determine the presetting. To simplify this determination, it is assumed that the curvature within each finished lot is roughly the same. In other words, the first pipe of a production lot is measured and, given the same pipe size (diameter, wall thickness), the same material and the same welding parameters, its curvature is used for all pipes of the production lot.